Proteinogenic amino acids are
amino acids that are
precursors to
proteins, and are incorporated into proteins during
translation. Throughout known
life, there are 23 proteinogenic amino acids, 20 in the standard
genetic code and an additional 3 that can be incorporated by special translation mechanisms. Both
eukaryotes and
prokaryotes can incorporate
selenocysteine into their proteins via a nucleotide sequence known as a
SECIS element, which directs the cell to translate a nearby UGA
codon as
selenocysteine (UGA is normally a
stop codon). In some
methanogenic prokaryotes, the UAG codon (normally a stop codon) can also be translated to
pyrrolysine. In
bacteria, the AUG
initiation codon is translated to
N-formylmethionine when it is actually used to initiate translation and translated normally (to
methionine) at other times. In eukaryotes, there are only 21 proteinogenic amino acids, the 20 of the standard genetic code, plus selenocysteine. Humans can synthesize 12 of these from each other or from other molecules of intermediary metabolism. The other nine must be consumed (usually as their protein derivatives), and so they are called
essential amino acids. The essential amino acids are
histidine,
isoleucine,
leucine,
lysine,
methionine,
phenylalanine,
threonine,
tryptophan, and
valine (i.e. H I L K M F T W V).